Reference is made to copending patent application, filed simultaneously herewith in the name of Dawes et al. and entitled xe2x80x9cWAVEGUIDES AND METHOD OF MAKING THEM.xe2x80x9d
1. Field of the Invention
This invention relates to an optical material suitable for making optical waveguide devices and a method of manufacturing optical waveguide devices.
2. Technical Background
Wavelength division multiplexers (WDMs) are designed to separate broad wavelength bands comprised of many discrete narrow band optical signals (individual channels corresponding to different signal streams) into a number of predetermined narrow wavelength bands each corresponding to an individual signal channel, at designated output locations. An example of wavelength division multiplexer is a phase array device formed in silica based glass. When subjected to changes in operating temperature, the phase array device shifts the channels into incorrect output locations. The temperature dependent shifts of the channels are caused by index of refraction changes in optical glass, which result in variations in optical path length (OPL) in the phase array.
More specifically, the phase arrays rely on designed OPL differences to provide a grating effect and to separate, based on wavelength, single broadband input light into several narrow band channels. In such devices, the temperature dependence of the center channel""s central wavelength""s position arises from the OPL shifts or changes with temperature, which is due to non zero CTE (coefficient of thermal expansion) and the dn/dT (temperature induced refractive index change) of the glass. The central wavelength of the center channel may vary by as much as 0.01 nm/Cxc2x0. If the channel spacing were 0.5 nm, a 50-degree temperature shift would shift the center channel into an adjacent position. This could result in loss of channels and scrambling in subsequent channel routing. As a result, WDM specifications generally include a thermal stability requirement, allowing a center channel""s central wavelength shift of less than 0.05 nm over a 70-degree temperature range. In order to overcome the channel shift problem, and to provide the required thermal stability one can utilize a phase array with an actively controlled temperature packaging. However, this approach is relatively expensive and results in large size packaging.
Passive athermalization for phase arrays requires some form of correction for the temperature dependent OPL shifts. It is known to etch a gap in the phase array of a planar WDM device, creating a separation between the wave guide pairs, and then to fill this gap with an optically transmissive material that has a negative dn/dT. This approach is described in the article entitled xe2x80x9cAthermal silica based arrays waveguide grating multiplexerxe2x80x9d, Electronic Letters, volume 33, No. 23, pp. 1945-1946, 1997. The dimensions of the gap are such that the light propagating through each arm of the phase array is compensated by having to move through the negative dn/dT material, such that the overall thermally induced optical path length change is zero. However, this approach is also problematic. More specifically, the gap region (with the negative dn/dT material) is lost due to diffraction through the gap. Loss reduction by using multiple gaps is possible, as disclosed in OFC-98 Technical Digest TU 01-1, pg. 204-206, but this has the disadvantage of back reflections and potentially high crosstalk. Loss reduction by formation of slab waveguides, comprised of different index layers in the polymer material situated in the gap is also known. However, such waveguides are difficult and costly to manufacture and compensate only for half of the diffraction induced losses.
Fiber to fiber splicing is a critical process step in the fabrication of many devices, and is especially difficult to do when the thermo-mechanical properties of the two fibers are significantly different from one another. Conventional fusion splicing techniques can not be employed for example, for coupling a highly doped amplifying fiber to a silica transmission fiber because during the splicing process the fiber with the lower melting temperature will melt first and fuse to the high melting temperature fiber (silica fiber). When the splice cools down, significant thermal stress builds up in the joint and ultimately causes a fracture. Furthermore, the fabrication of a generic fiber-to-fiber splice requires the two optical fibers to be actively aligned with the ends separated by a distance of about 2 xcexcm to about 150 xcexcm. Such fibers are difficult to couple efficiently because the signal light beam provided by the input fiber expands before reaching the second, output fiber, resulting in significant optical loss.
Thus there is a need for devices that provide efficient light coupling between pairs of optical waveguides, when the two waveguides constituting each pair are separated from one another.
End-fire curing is the process of sending UV light through the waveguide so that it exits the waveguide and photo-cures (photo-polymerizes) the region which contains photo-polymerizable material and which is located adjacent to the exit face of the waveguide. Such photo-polymerizable materials are one or more monomers with similar diffusion coefficients. These materials tend to further polymerize after the initial exposure, when subsequently exposed to thermal or photo radiation. These materials are susceptible to changes in their index of refraction with time. Thus, there is a need for better photo-curable adhesives.
According to one aspect of the present invention, a UV light-curable composition comprises: (a) a first component, said first component being UV light-polymerizable polymer having a first index of refraction; and (b) a second component, the second component being UV light-polymerizable monomer having a second index of refraction, the second index of refraction being higher than said first index of refraction; wherein the first component polymerizes slower upon exposure to UV radiation than the second component. According to one embodiment, the photo-curable composition includes: (a) fluorinated maleimide/fluorinated acrylate/glycidyl methacrylate polymer having glass transition temperature over 150xc2x0 C. after cationic curing from UV radiation, 20-50% wt percent; (b) diacrylate or dimethacrylate monomer, 35 to 65 wt %; (c) glycidyl methacrylate monomer, 5-20 wt %; and (d) at least two photoiniators, 0.5 to 2%.
According to another aspect of the present invention, a waveguide device comprises: (i) at least one pair of waveguides located such that (a) light radiation propagating through one of these waveguides will be at least partially coupled to a corresponding waveguide and, (b) these waveguides are separated by a gap of about 2 xcexcm to about 500 xcexcm; and (ii) another waveguide connecting these pair of waveguides, wherein the gap contains photo-curable composition. The composition includes: (a) fluorinated maleimide/fluorinated acrylate/glycidyl methacrylate polymer having glass transition temperature over 150xc2x0 C. after cationic curing from UV radiation, 20-50% wt percent; (b) diacrylate or dimethacrylate monomer, 35 to 65 wt %; (c) glycidyl methacrylate monomer, 5-20 wt %; and (d) at least two photoiniators, 0.5 to 2%.
According to yet another aspect of the present invention, a method of making a coupling waveguide device comprises: (i) providing at least one pair of waveguides located such that (a) light radiation propagating through one of these waveguides will be at least partially coupled to a corresponding waveguide and (b) these waveguides are separated by a gap of about 2 xcexcm to about 500 xcexcm; (ii) filling the gap with a photo-polymerizable composition, the composition including (a) fluorinated maleimide/fluorinated acrylate/glycidyl methacrylate polymer having glass transition temperature over 150xc2x0 C. after cationic curing from UV radiation, 20-50% wt percent; (b) diacrylate or dimethacrylate monomer, 35 to 65 wt %; (c) glycidyl methacrylate monomer, 5-20 wt %; and (d) at least two photoiniators, 0.5 to 2%; (iii) providing photo-radiation through the gap. The photo-radiation photo-polymerizes said composition thereby creating a coupling waveguide between said pair of waveguides.
For a more complete understanding of the invention, its objects and advantages refer to the following specification and to the accompanying drawings. Additional features and advantages of the invention are set forth in the detailed description, which follows.
It should be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various features and embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.